Definition of ELA calculation

ELA calculation must be done in different flight phases because electrical loads are different depending on the phase of the flight. This is defined and required in aviation authority’s regulations. (EASA 2017.) Ground phase is defined basically when all wheels are in contact with ground, engines are not running and aircraft is connected to GPU or powered by APU. In A350 aircraft, the ground phase consists of 6 sub-phases varying from 0 to 90 minutes due to the reason that electrical loads are different depending on the phase in ground operation. (Airbus 2017b.)

Flight phases are divided into eight different phases:

 Start phase is from start of the engines to start of roll

 Roll phase begins when aircraft is leaving the gate and taxiing to runway in order to start take-off roll. The roll sequence lasts until the landing gear is not compressed any more.

 Take-off phase begins from the moment the aircraft rises from the surface and lasts to altitude of 1500 feet.

 Climb starts at 1500 feet ending at aircraft cruise level or stabilized level

 Time window of cruise phase is from the end of climb to start of descent.

Major part of the flight is covered normally with cruise phase. Electrical consumption is in the highest level in this phase because all galley components, like ovens and coffee makers, are energized during this period.

 Descent starts when aircraft leaves the cruising altitude and ends at 800 feet.

 Landing phase commences at 800 feet when landing gear is in down position until touchdown and rollout. The aircraft is at low speed under control and the landing phase ends when aircraft is ready to start taxiing.

 Taxi phase is from touchdown to the moment when the engines are not running and aircraft is parked. (Airbus 2017b.)

According to A350 delivery ELA (Airbus 2017b), the total electrical loads, described in figure 4, show that loads vary in different flight phases. In cruise phase, the power demand is higher than in the descent phase because galley inserts, like ovens, are being energized. The dark blue color shows the not sheddable power demand in kVA and the light blue color the sheddable power demand in kVA.

Figure 4. A350 total electrical loads in different flight phases (Airbus 2017b).

ELA analysis is achieved by evaluating and calculating operational and maximum values in different flight phases. A calculation method must be used to evaluate post-delivery modifications, such as Airbus service bulletin (SB), to confirm that the new modification will be within the limits of electrical power sources and network capabilities. In the A350 aircraft, ELA calculation must be made in four different levels (figure 5). These levels are:

 Power source level, which consists of the generator, emergency generator and batteries. This part gives breakdown of the busbar loads for each main power source like VFG.

 Converter level, which consists of TR and ATU.

 Electrical distribution level consisting of busbars. This part gives the nominal power rating and the maximum and operational loads at each C/B, RCCB and SSPC connected to a specified busbar for the different ground and flight phases.

 Grounding and bonding level consisting of the electrical structure network.

This calculation is only made by the aircraft manufacturer. (Airbus 2016.)

Figure 5. Electrical load levels (Airbus 2016).

In A350 aircraft, the airframe is not a metallic structure anymore like in classic aircrafts. Carbon material is used for the structure, including aircraft skin. Two metallic networks, ESN and MBN, ensure proper functioning of the electrical bonding and electrical grounding. This network of current return (ESN) has to be taken into account when calculating ELA in A350 aircraft. (Guadalupe et al. 2016, p. 401.)

According to Airbus electrical load analysis manual, the following assumptions and design criteria must be used when updating and calculating ELA. The load demand is calculated for a stabilized situation after the first five minutes of operation.

Equipment of the aircraft system that operates in stabilized condition has PF (power factor) close to 1. Consequently, calculations with true power factors would not significantly reduce loads at the VFG level, and loads described in the ELA are the result of arithmetical calculations based on assumption that PF is equal to 1. (Airbus 2016.)

The power consumption types are nominal power, operational consumption and maximum consumption. Nominal power is specified as power consumption under 230V or 115V for AC loads and 28V for DC loads at their input. In addition, nominal power can be the sum of several equipment loads and used for the selection of the protection rating and for wiring sizing. The operational and maximum load values are ratios of nominal power, and they are computed by multiplying nominal power by the duty cycle and the simultaneous use ratio, where:

 Duty cycle is the time during which the component or system is supplied / time during component or system is not supplied.

 Simultaneous use ratio is the value based on flight conditions and number of passenger. (Airbus 2016.)

Operational permanent consumption corresponds to the most probable power consumption in normal operating conditions, and it is used for calculating electrical load analyses in degraded configuration, for example, in loss of electrical power sources. Maximum permanent consumption corresponds to the most probable power consumption in the most unfavorable conditions, and it is used for calculating electrical load analyses in normal configuration. (Airbus 2016.)

The classification of permanent and intermittent loads is described in table 1.

Considering overload capabilities at the power source level, the loads are classified as permanent and intermittent based on their duration of operation.

Table 1. Permanent and intermittent load classification (Airbus 2016).

Load type < 30 seconds 30-300 seconds >300 seconds

AC load INT INT PERM

DC load INT PERM PERM

Certain aircraft systems consume lots of electrical power or are critical to the safe flight management, such as air-conditioning and pneumatic system, auto flight system, galley loads, fire-detection system, fuel system, lightning system, flight control system and ice and rain protection system. When calculating ELA, duty cycles of these systems must be considered.

Electrical load for the air-conditioning and for pressurization is based on a system having two air conditioning packs and 8 zones of temperature control. Automatic and manual override electrical control is provided for the system. In general, these loads operate continuously at 100% load throughout ground phase and all flight phases. Electrical power is also provided for cabin recirculation fans. They are assumed to operate continuously except during single VFG operation. When the

right circulation fan is shed, then the electrical power for the forward equipment cooling fans is provided. One fan is operating during all operational conditions.

Entries for two aft ventilation fans are provided but only one will normally run.

Separate heater and fan are used for heating the aft cargo compartment with 70 % duty cycle. (UTC 2015.)

Generally, automatic flight system loads operate at 100 % from engine start through landing. During category III Autoland operation (low visibility approach and landing), the relevant loads, normally connected to the AC busbar, will be connected to the inverter/TR in order to confirm continuous power in case of AC load interruption. Communication loads include aircraft transmitter and receiver equipment, passenger address, cockpit voice recorder and passenger entertainment equipment like IFE. Operation of the radios is based on 10 % of the total operating time on transmit and 90 % of the time in monitoring mode. (Airbus 2017c.)

During airplane loading, with only one GPU available, 40 % galley load is allowed.

On the other hand, during cruise phase when all four VFG generators are operating, 100 % galley load is available. During descent and landing, 10 % galley load demand is available. Automatic load shedding of non-essential circuits will de-energize galley loads during degraded operations in flight. (UTC 2015, Airbus 2017c.) Fire, overheat and smoke detection systems are operated in standby and in monitoring mode throughout the ground and flight phases. Actuation of the detection systems and fire extinguishers is not included for the normal flight operation. (Airbus 2017c.)

Electrical load for the fuel system assumes the use of six fuel pumps during all flight phases. One fuel pump is connected to the ground service busbar to supply fuel for continuous ground operation of the APU. The remaining pumps are supplied from the main AC busbars. DC fuel pump supplied from the battery busbar is provided

for APU start-up when the AC power is not available. AC power is assumed available during all flight modes. The indication and recording system consists of engine indication, flight data recording and flight warning systems, and they operate at 100 % utilization. (Airbus 2017c.)

In the aircraft lightning system, anti-collision lights are operated continuously at 100 % during engine start and throughout the flight. Nose gear landing lights and wing landing lights are operated full time only during take-off and landing. Runway turn-off lights are operated during taxi and landing only. Electrical loads for the navigation systems, including air data systems, heading, attitude and weather radar, are 100 % in all flight phases. The oxygen pressure indication system is on during all flight phases. In the pneumatic system, continuous power is required for air supply status indication and duct pressure transmitters. (Airbus 2017c.)

In the electrical power system, there are no indicated electrical loads for the GCU when the associated VFG source (engine) is operating. The GCUs receive primary control power from the AC system VFGs when the latter are rotating. Backup control power is provided to each GCU from the hot battery busbars. In the flight control system, electrical loads related to control and position indication of the flight control surfaces are operated continuously at 100 % during engine start and throughout the flight. Trim actuators and hydraulic valves are all momentary, and alternate flap and slat control motors are used only as a back up to the primary system. (UTC 2015.)

In ice and rain protection, window heaters are used for de-icing and de-fogging of the cockpit windows. When switched on, heating power for the front windows is thermostatically modulated by temperature sensors. The load has been estimated at approximately 25 % full load on the ground, 50 % full load during take-off and landing and 90 % full load during cruise. The electrical load for the multi-function

probes is based on the installation of 3 probes. These loads are self-modulated by the characteristic resistance of the heater element. The utilization of power is estimated at approximately 25 % during ground operation and 65 % during take-off, cruise and landing. (Airbus 2017c.)